Method for coating a substrate

ABSTRACT

A method of coating a substrate is proposed, in which a starting material in the form of a process beam is sprayed onto a surface of the substrate by means of thermal spraying, wherein the surface of the substrate is initially pretreated with a plasma flame of a plasma spray device without material deposition and the process beam including the starting material is subsequently applied onto the surface.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a National Stage of International Application No. PCT/EP2012/074594, filed on Dec. 6, 2012, and published in German as WO/2013/083671 A1 on Jun. 13, 2013. This application claims the benefit and priority of European Application Nos. 11192846.1, filed on Dec. 9, 2011 and 12168281.9, filed on May 16, 2012. The entire disclosures of the above applications are incorporated herein by reference.

BACKGROUND

This section provides background information related to the present disclosure which is not necessarily prior art.

1. Technical Field

The invention relates to a method of coating a substrate.

2. Discussion

Nowadays, thermal spray methods are frequently used for the coating of substrates, such as, for example, plasma spraying; high velocity flame spraying (HVOF), light arc spraying and/or light arc wire spraying.

Layers are generated on a substrate using these methods, wherein the substrate is, for example, metallic and the coating is metallic, ceramic or also a mixture of both. Frequently, the coating includes a plurality of individual layers which are arranged on top of one another and as a rule have different functionalities.

So, for example, thermal barrier coatings (TBC) are known, as are corrosion protective layers, erosion protective layers or also gliding layers which simplify the sliding of counter running bodies against one another. In this context an example of application is the coating of cylinder running surfaces in combustion engines having layers which have good lubrication properties and good friction properties, whereby the running properties of the piston in the cylinder are improved. There are also applications in which a deliberately rough layer is applied by means of thermal spraying. For example, prostheses, such as hip joint prostheses, which are typically made from the biocompatible metal titanium, are thus subsequently provided with a rough coating in order to simplify a growth of the bone tissue at the prosthesis. A ceramic substance, for example, a hydroxyl apatite is generally used in this connection as a coating material.

It is striven for, in all applications that the layer produced by means of thermal spraying adheres as well as well and as permanent as possible at the substrate. For this purpose the surface to be coated is generally subjected to a pretreatment prior to the thermal spraying which pretreatment is also referred to as activation. This activation can, for example, take place by sand blasting, corundum blasting, blasting with gray cast iron, high pressure water beams, diverse laser methods or other known activation methods.

As a rule the pretreatment is very demanding in time and also requires special substances, such as corundum, sand or, for example, high pressure water beams. An additional problem results therefrom that for many activation methods, for example, on corundum blasting, additional cleaning steps or drying steps are necessary before the thermal spraying can begin. Furthermore, despite careful cleaning it can happen that sand particles or corundum particles remain at the surface and remain in the layer on the subsequent coating thereof. Such foreign bodies can later lead to undesired formations of cracks which could have serious consequences, for example, in the case of coated hip joint prosthesis. However, even when no corundum particles remain at the surface, the surface roughness, in particular of high strength substances, generated by means of the roughening process can lead to the formation of fatigue cracks.

SUMMARY OF THE INVENTION

Starting from this prior art, it is therefore an object of the invention to provide a method of coating a substrate by means of thermal spraying, in which the pretreatment or the activation of the surface to be coated takes place very simply, wherein a contamination of the surface to be coated should be avoided as well as possible.

In accordance with the invention a method of coating a substrate is thus provided in which a starting material in the form of a process beam is sprayed onto a surface of the substrate by means of thermal spraying, with the surface of the substrate initially being pretreated with a plasma flame of a plasma spray device without material deposition and the process beam including the starting material being subsequently applied onto the surface.

It has surprisingly been found that a very good adherence of the subsequently applied layer can be achieved by means of a pretreatment or an activation with the plasma flame. Since the plasma flame does not include any particles or foreign bodies on pretreatment also a contamination of the surface to be coated is excluded. Therefore, demanding cleaning steps or drying steps are disposed with following the activation. The plasma spray device is substantially operated with the same parameters as they are typically used for the coating by means of plasma spraying during the pretreatment. Only no starting material is introduced into the plasma flame.

The method in accordance with the invention is in particular suitable also for such applications in which the substrate is metallic.

The starting material is preferably a metallic material or a ceramic material or a mixture of such materials as thereby the desired properties of the coating can be matched very well to the respective case of application. In particular, also metal matrix compound materials, such as MMC or cermet are meant hereby.

In accordance with a preferred embodiment, the thermal spraying is a plasma spraying.

The method in accordance with the invention is in particular suitable also for thermal spraying in the low pressure region, which means that the spraying is carried out at a process pressure which is smaller than the atmospheric pressure. Therefore, the invention is also specifically suitable for vacuum plasma spraying (VPS) or for low pressure plasma spray processes, such as LPPS (Low Pressure Plasma Spraying) or LPPS-TF (LPPS Thin Film).

In a preferred embodiment a thermal spray apparatus for generating the process beam and the plasma spray device are commonly moved relative to the surface of the substrate separate from one another in time or space, such that the plasma flame initially pretreats a region of the surface and after a predeterminable time frame, the process beam is subsequently applied onto the same region. This measure enables a particularly simple method procedure which is of little demand in effort and cost, since the pretreatment and the thermal spraying take place successively one after the other without different intermediate steps or a change of an apparatus to another being required. The thermal spray apparatus and the plasma spray apparatus can, for example, be mounted at a common axis or at a common arm, for example, at the arm of a treatment robot. The two apparatuses are, for example, mounted downstream of one another at a predefinable spacing at the arm, so that for a linear movement of the arm initially the spray apparatus and subsequently the thermal spray device pass the same region of the surface to be coated.

A different variant is that, for a plasma spray device rotatable about a shaft axis, such as for example, the plasma torch which is obtainable from Sulzer Metco AG (Switzerland) under the reference F210, a second plasma torch is arranged such that it is located back to back with the first plasma torch, i.e. displaced by 180 degrees with regard to the circumferential direction. The first plasma torch initially passes a region for pretreatment and subsequently the second plasma torch for coating passes the same region on a rotation. Such rotatable plasma torches are, for example, used for the coating of curved surfaces, such as cylinder running surfaces of combustion engines.

In a different preferred embodiment, the surface of the substrate is pretreated without material deposition by the plasma spray device and the process beam is subsequently generated for the coating with the same plasma spray device. In this embodiment, thus only one plasma spray device is required which is why this embodiment is particularly simple and economic. By means of this plasma spray device, the pretreatment is initially carried out, wherein the supply of the starting material is switched off. Subsequently the supply of the starting material is switched on and the previously pretreated region is coated.

In practice it has been shown that it is advantageous when the plasma flame for pretreating is generated using substantially the same plasma parameters, as the plasma flame for generating a process beam, in particular using the same current, the same gas and the same gas flow rate.

From a practical aspect it is advantageous when the spacing between an exit nozzle of the plasma spray device and the surface of the substrate are of equal size for the pretreatment and the application of the process beam.

It is also been found favorable when the time span between the pretreatment and the application of the same region with the process beam (2) amounts to at most five minutes.

It has been found to be particularly advantageous when the time span between the pretreatment and the application of the same region with the process beam (2) amounts to at most one minute.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following the invention will be explained in detail with reference to embodiments and with reference to the drawing. In the schematic drawing there is shown partially in section:

The drawings described herein are for illustrative purposes only of selected embodiments and not all possible implementations, and are not intended to limit the scope of the present disclosure.

FIG. 1 a schematic illustration of an apparatus for carrying out a first embodiment of a method in accordance with the invention; and

FIG. 2 a schematic illustration of an apparatus for carrying out a second embodiment of a method in accordance with the invention.

Corresponding reference numerals indicate corresponding parts throughout the several views of the drawings.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Example embodiments will now be described more fully with reference to the accompanying drawings.

The method in accordance with the invention for coating a substrate 10 by means of thermal spraying is, in particular, characterized thereby that the surface of the substrate is initially pretreated with a plasma flame. The invention is suitable for all thermal spray methods, such as, for example all plasma spray methods, high velocity flame spraying (HVOF), light arc spraying and/or light arc wire spraying. In this connection the term thermal spraying also includes such spray processes in which the process gas is “cold” in comparison to classical plasma spraying, for example, only has a few hundred Kelvin. These processes in which the particles for the coating primarily adhere at the substrate due to their kinetic energy are typically referred as cold gas spraying or kinetic gas spraying.

In the following reference is made to the case of application important for practice having exemplary character, in which the thermal spraying is a plasma spraying. Hereby, it can both be a plasma spray process at normal pressure or at atmospheric pressure (APS: Atmospheric Plasma Spraying) and also be a vacuum plasma spray process or low pressure plasma spray process, such as the already mentioned VPS process, LPPS process or LPPS-TF process.

FIG. 1 shows a plasma spray apparatus which is generally referred to using the reference numeral 1 and is suitable for carrying out a method in accordance with the invention in a very schematic illustration. Furthermore, a substrate 10 is illustrated schematically in FIG. 1 at which a coating in the form of a layer 11 is deposited.

The method in accordance with the invention preferably includes a plasma spraying, the kind of which is described in accordance with WO-A-03/087422 or also in U.S. Pat. No. 5,853,815. This plasma spray method is a thermal spraying for manufacturing a so-called LPPS-thin film (LPPS=Low Pressure Plasma Spraying).

The plasma spray apparatus 1 illustrated in FIG. 1 includes a plasma spray device 3 known per se, as a thermal spray apparatus, having a plasma torch for generating a plasma which is not illustrated in detail. In this respect it can, for example, be a plasma spray device of the type F4 which can be obtained from Sulzer Metco AG (Switzerland). In a manner known per se a process beam 2 is generated with the plasma spray device 3 from a starting material P a process gas mixture G and an electrical energy E. The introduction of these components E, G and P is symbolized by the arrows 4, 5, 6 in FIG. 1. The generated process beam 2 exits via an exit nozzle 7 and transports the starting material P in the form of a process beam 2 in which the material particles 21, 22 are dispersed in a plasma. This transport is symbolized by the arrow 24. The different material particles 21, 22 should indicate that it is generally possible that the starting material P can include several different material particles, this must naturally, however, not be the case. The material particles 21, 22 are generally powder particles. In the case of wire-based method such as, for example, PTWA (Plasma Transferred Wire Arc), the starting material is present as a wire.

The plasma spray apparatus 1 further includes a second plasma spray device 3′ which can be of the same type like the plasma spray device 3 but does not have to be. This second plasma spray device 3′ serves thereto to generate a plasma flame 2′ from a process gas mixture G′ and electrical energy E′ which exits through an exit nozzle 7′ and which can be applied to a region B of the surface of the substrate 10 to be coated. The introduction of the components E′ and G′ is symbolized by the arrows 4′ and 5′ in FIG. 1.

The two plasma spray devices 3 and 3′ are mounted on a common arm 8 such that they are fixed relative to one another at a predefinable spacing A. The arm 8 can, for example, be movable by a non-illustrated treatment robot, as is indicated by the arrow V in FIG. 1. In the present embodiment the two plasma spray devices are attached at the arm 8 so that the spray distance, this means the spacing D between the exit nozzle 7 and the substrate 10 is of the same size as the spacing between the exit nozzle 7′ of the second plasma spray device 3′ and the substrate 10, this means the pretreatment takes place at the same spacing between the plasma spray device 3′ and the substrate 10 like the thermal spraying. Depending on the application it can naturally also be more favorable to select between different spacings between the respective plasma spray device and the surface to be coated for the pretreatment and for the plasma spraying.

In the most cases of application it is also favorable when the plasma parameters for the generation of the plasma flame 2′ are substantially the same as the plasma parameters for generating the process beam 2. Primarily, the current for generating the plasma, as well as the composition and the flow rate of the process gas G and or G′ are meant with the plasma parameters.

For coating of the substrate 10 it is now proceeded as follows. The two plasma spray devices 3, 3′ are activated so that the plasma spray device 3 generates the process beam 3 and the second plasma spray device 3′ generating the plasma flame 2′. It can possibly be advantageous to activate the plasma spray device 3 for generating the process beam 2 a little later than the plasma spray device 3′ for the pretreatment, as the latter has a certain advance. Now the two plasma spray devices 3 and 3′ are moved past the surfaces of substrate 10 to be coated at a spacing D as it is indicated by the arrow V in FIG. 1. Now the surface of the substrate 10 is pretreated in the region B by means of the plasma flame 2′ of the plasma flame device 3′. In this connection no material deposition takes place, but merely the application of the plasma flame 2′. Due to the common movement of the two plasma spray devices 3 and 3′ the plasma spray device 3, which generates the process beam 2, arrives at the just pretreated region B at a predefinable time span later than the pretreatment occurred by means of the plasma flame 2′. This time span depends on the velocity with which the two plasma spray devices 3, 3′ are moved and on the spatial spacing A of the two plasma spray devices 3, 3′. In this context the time span typically lies in the region of up to a few ten seconds.

When the plasma spray device 3 has passed the previously pretreated region B a process beam 2 including the starting material P is applied to this in a manner known per se, whereby the layer 11 is generated on the substrate 10.

It has been shown that a very good adherence of the layer 11 at the substrate 10 can be realized without a previous treatment with corundum beams, sand beams, high pressure water beams or the like being necessary for this purpose due to this pretreatment by means of plasma flame 2′. Bond strength of at least 40 MPa can be achieved by means of the method in accordance with the invention.

The method in accordance with the invention is naturally also suitable for multi-layer systems, for example, a bond layer or a bond promoting layer can initially be applied on the substrate 10 using the method in accordance with the invention onto which one or more further layers are subsequently sprayed.

The substrate 10 can be a metallic substrate or also be composed of a ceramic, of a plastic or of a mixture of these materials.

All materials are suitable as a starting material P which can be used in a thermal spray process. In the case of plasma spraying as a thermal spray process the starting material P is generally present in the form of a powder which is conveyed by means of a support gas in the plasma flame.

A variant of the apparatus illustrated in FIG. 1 consists therein that the two plasma spray devices 3 and 3′ are not arranged at a common arm 8, but that they can be moved independent from one another by independent movement apparatuses, for example, two treatment robots can be provided, each of which moves one of the two plasma spray devices 3 or 3′. It can then be realized that the plasma spray device 3 for the generation of the process beam 2 follows the plasma spray device 3′ for generating the plasma flame 2′ for the pretreatment at a predefinable spacing via the control of the two movement apparatuses, so that initially the pretreatment takes place at the region B and then after a predefinable time span the coating takes place. This variant has the advantage that also more complex, in particular also curve-like movements of the plasma spray device 3, 3′ can be carried out.

A further variant consists therein that only one plasma spray device, for example, the plasma spray device 3 in FIG. 1, is used for carrying out the method. For this purpose the introduction 6 of the starting material B is switched off for the pretreatment of a region B, so that the plasma flame is initially only applied to the region B. The introduction 6 of the starting material P is subsequently activated, the just pretreated region is coated with the process beam 2. This variant with only one plasma spray device can either be carried out such that initially the overall surface to be coated is pretreated with the plasma flame and subsequently the overall surface is coated with the process beam 2 depending on the size of the substrate 10 to be coated. However, it is also possible to initially only pretreat a region, for example, a strip of the surface to be coated with the plasma flame, to then coat this region with the process beam 2, subsequently to pretreat the next region and then to coat this and to carry on with this region-wise for so long until the overall surface of the substrate 10 is provided with the layer 11.

In practice it has been shown that the timely separation between the pretreatment and the plasma flame and the thermal spraying can amount from to a few seconds up to a few minutes. This naturally also depends on the substrate to be coated. It is generally advantageous when the timely separation does not exceed five minutes and preferably a minute.

As a specific case of application for the method in accordance with the invention the coating of aluminum or grey cast iron in the bores of cylinders of a combustion engine are mentioned in this context, whereby a cylinder running surface having very good lubrication properties, friction properties and running properties can be generated.

A different example of application is the coating of prostheses which are manufactured from titanium, with hydroxyl apatite. In this connection the omission of a previous beam with corundum or sand in particular is a very essential advantage.

FIG. 2 shows a schematic illustration of an apparatus for carrying out a second embodiment of a method in accordance with the invention. In this connection it is a rotatable plasma spray device, for example, an apparatus which is a combination of the plasma torch distributed by the Sulzer Metco AG (Switzerland) under the reference F 210 with the RotaPlasma unit distributed by the same company.

In the following only the differences to the embodiment explained with reference to FIG. 1 will be explained in detail. Otherwise, explanations made in connection with FIG. 1 or with reference to FIG. 1 apply in a corresponding manner also for the embodiment explained by means of FIG. 2. In particular the reference numerals have the same meaning.

The plasma spray apparatus 1 illustrated in FIG. 2 only includes one plasma spray device 3. The substrate 10 in this example is a cylinder bore whose curved inner surface should be provided with the layer 11 as a coating. In a manner known per se the plasma spray device 3 (torch) for generating the process beam 2 or the process flame 2′ is provided at a torch shaft 30 of the plasma spray apparatus 1. The plasma spray device 3 for coating the curved inner surface of the substrate 10 is rotatably arranged around a shaft axis C. In this connection the torch shaft 30 itself rotates, as is indicated by the arrow U in the example of FIG. 2. Furthermore, the torch shaft 30 is linearly movable in the direction of the shaft axis C, i.e. movable up and down in accordance with the illustration, so that the overall inner surface of the cylinder bore can be coated by the rotation about the shaft axis C and the up and down movement of the plasma spray device 3.

For carrying out this embodiment of the method in accordance with the invention it is proceeded as follows. Initially the plasma spray device 3 is activated, wherein the introduction 6 of the starting material P is not yet switched on, so that the plasma spray device 3 generates a plasma flame which does not include coating material. Now the overall inner surface of the cylinder bore serving as the substrate 10 is pretreated with the plasma flame, due to the rotation U about the shaft axis C and the upward movement and/or the downward movement of the plasma spray device 3 in accordance with the illustration. In this connection the upward and downward movement of the plasma spray device 3 can take place a plurality of times in the cylinder bore, for example, three times. Subsequently, the plasma spray device is moved out of the cylinder bore or at its upper end in accordance with the illustration and the introduction 6 of the starting material P is activated, so that the plasma spray device 3 now generates the process beam 2. The layer 11 is now applied onto the substrate 10 by means of this due to a one time or a several time up and down movement of the plasma spray device 3 for a simultaneous rotation U about the shaft axis C.

The starting material P is injected into a plasma defocusing the material beam and is therein partially molten or completely molten or at least made plastic at a low process pressure which amounts to at most 10 000 Pa and preferably to at most 1000 Pa in the LPPS-TF process described in this context. At least a part of the starting material is also evaporated or transferred to the vapor phase depending on the process procedure in this connection in particular during the LPPS-TF process. For this purpose a plasma having a sufficiently high specific enthalpy is generated, so that a very dense and thin layer 11 is generated at the substrate. The variation of the structure is substantially influenced and controllable by the coating conditions, in particular by the process enthalpy, by the work pressure in the coating chamber, as well as by the process beam. Therefore, the process beam 2 has properties which are determined by the controllable process parameters.

A variant of this method consists therein to provide a second plasma spray device at the torch shaft 30 which is arranged displaced to the plasma spray device 3 with regard to the circumferential direction of the torch shaft 30, for example displaced by 180°, so that the two plasma spray devices are arranged back to back. Then the second plasma spray device for generating the plasma flame is used for the pretreatment and the other plasma spray device for generating a process beam 2 is used in a manner corresponding to the same manner as was explained in connection with FIG. 1. On a rotation U of the torch shaft 30 the plasma beam initially passes a region for the pretreatment and half a turn later the process beam 2 passes this pretreated region. In this connection it is particularly advantageous in practice, when the two plasma spray devices are additionally displaced with regard to the axial direction, i.e. are arranged at different heights.

In the following examples for the pretreatment by means of the plasma flame are still being provided, in which a cylinder bore is respectively being coated. The coating—i.e. the pretreatment by means of the plasma flame and the subsequent thermal spraying—are each carried out with a plasma spray device of the type F210 of Sulzer Metco AG (Switzerland) as an atmospheric plasma spray process as was explained in accordance with FIG. 2.

In this connection the number of repetitions respectively indicate how frequently the plasma spray device 3 was moved upwardly and downwardly in the cylinder bore, in this connection a repetition is a complete downward and upward movement.

EXAMPLE 1

The pretreatment took place with the following plasma parameters:

Current: 400 A

Process gas: argon 60 SLPM (standard liters per minute), hydrogen 5 SPLM, nitrogen 4 SPLM.

Spacing D between the plasma spray device 3 and the surface to be coated: 30 mm

Number of repetitions: 3

Thereafter a bond layer of the nickel-based alloy Diamalloy 4008NS of Sulzer Metco AG (Switzerland) is initially applied at a spacing D of 30 mm and a layer of a low alloy steel, to which 30 wt.-% molybdenum is admixed, is applied thereon as a cover layer at a spacing of 45 mm in a manner known per se. In this connection the steel and the molybdenum are not present as an alloy in the starting material as a powder mixture. The thereby generated coating shows a very good adhesion.

EXAMPLE 2

The pretreatment took place with the following plasma parameters:

Current: 410 A

Process gas: argon 60 SLPM (standard liters per minute), hydrogen 5 SPLM, nitrogen 4 SPLM.

Additionally a cover gas for covering the plasma flame with 16 SLPM is used.

Spacing D between the plasma spray device 3 and the surface to be coated: 45 mm

Number of repetitions: 3

In this example no bond layer is applied, but rather a layer of the material F4301 of Sulzer Metco AG (Switzerland) is directly applied at a spacing of 45 mm as a cover layer in a manner known per se onto the surface pretreated with the plasma flame.

The thereby generated coating shows a very good adhesion.

EXAMPLE 3

The pretreatment took place with the following plasma parameters:

Current: 550 A

Process gas: argon 60 SLPM (standard liters per minute), hydrogen 6 SPLM, nitrogen 4 SPLM.

Additionally a cover gas for covering the plasma flame with 16 SLPM is used.

Spacing D between the plasma spray device 3 and the surface to be coated: 45 mm

Number of repetitions: 3

In this example no bond layer is applied, but rather a layer of the material F2071 of the Sulzer Metco AG (Switzerland) is subsequently directly applied in a manner known per se as a cover layer at a distance of 45 mm onto the surface pretreated with the plasma flame. This material is a metal matrix compound material having a mixture of 65 wt.-% of a corrosion-resistant steel and 35 wt.-% of a ceramic, for example, Al₂O₃/ZrO₂ at a ratio of 80 to 20.

The thereby generated layer shows a very good adhesion.

The foregoing description of the embodiments has been provided for purposes of illustration and description. It is not intended to be exhaustive or to limit the disclosure. Individual elements or features of a particular embodiment are generally not limited to that particular embodiment, but, where applicable, are interchangeable and can be used in a selected embodiment, even if not specifically shown or described. The same may also be varied in many ways. Such variations are not to be regarded as a departure from the disclosure, and all such modifications are intended to be included within the scope of the disclosure. 

1. A method of coating a substrate in which a starting material in the form of a process beam is sprayed onto a surface of the substrate by means of thermal spraying, wherein the surface of the substrate is initially pretreated with a plasma flame of a plasma spray device without material deposition and the process beam including the starting material is subsequently applied onto the surface.
 2. The method in accordance with claim 1, in which the substrate is metallic.
 3. The method in accordance with claim 1, in which the starting material is a metallic material or a ceramic material.
 4. The method in accordance with claim 1, in which the thermal spraying is a plasma spraying.
 5. The method in accordance with claim 1, which is carried out at a process pressure which is smaller than the atmospheric pressure.
 6. The method in accordance with claim 1, in which a thermal spray apparatus for generating the process beam and the plasma spray device are commonly moved relative to the surface of the substrate separate from one another in time or space, such that the plasma flame initially pretreats a region of the surface and subsequently after a predeterminable time frame, the process beam is subsequently applied onto the same region.
 7. The method in accordance with claim 1, in which the surface of the substrate is pretreated by the plasma spray device without material deposition and the process beam is subsequently generated for the coating with the same plasma spray device.
 8. The method in accordance with claim 7, in which the plasma flame for pretreating is generated using substantially the same plasma parameters as the plasma flame for generating the process beam, in particular using the same current, the same gas and the same gas flow rate.
 9. The method in accordance with claim 7, in which the spacing between an exit nozzle of the plasma spray device and the surface of the substrate for the pretreatment and the application of the process beam are of equal size.
 10. The method in accordance with claim 1, in which the time span between the pretreatment and the application of the same region with the process beam amounts to at most 5 minutes.
 11. The method in accordance with claim 1, in which the time span between the pretreatment and the application of the same region with the process beam amounts to at most one minute. 